A styrenic polymer having a high degree of syndiotactic configuration is excellent in heat resistance, solvent resistance and electrical characteristics and accordingly, is expected to find a variety of use especially as a favorable raw material for use in the field of electrical insulation such as electronic machinery parts, printed circuit board, insulating self-adhesive tape and capacitor.
However, conventional styrenic polymers having a syndiotactic configuration sometimes fail to exert sufficient withstand voltage, for example, when molded into a thin film or the like, and thus further improvement in withstand voltage is desired for such styrenic polymers. As a method of improving the withstand voltage of such styrenic polymers having a syndiotactic configuration, there is proposed for example, in Japanese Patent Application Laid-Open No. 124750/1991, a method of decreasing residual catalyst and residual monomers in the resultant polymer. Although some improvement in dielectric breakdown voltage of the styrenic polymers is observed to a practicable extent according to the above-mentioned method, the method has involved the problem that its availability is limited in use requiring a high withstand voltage to direct current such as a printed circuit board and a capacitor. There is also proposed a composition containing a styrenic polymer having a syndiotactic configuration incorporated with a phosphorus-based antioxidant, phenol-based antioxidant, sulfur-based antioxidant or the like, but improvement in withstand voltage according to the aforesaid composition has still been insufficient.
In addition, it is reported that the withstand voltage of polyethylene, etc. is improved by the addition of a polycyclic aromatic compound such as pyrene, but the addition thereof to a styrenic polymer having a syndiotactic configuration can not attain such effect. Likewise, any of a method wherein a crosslinked polyethylene is employed, a method wherein a fine-grained crystalline structure is employed and a method in which crystallinity is improved each being known as a method of improving the dielectric breakdown voltage of polyethylene and the like is difficult to apply to a styrenic polymer having a syndiotactic configuration because of difference between the two with regard to the behavior in crystallization, crystalline structure, processing method and processing conditions.